There is an increasing demand further to reduce the energy consumption of reproduction apparatuses, e.g., photocopiers. In reproduction apparatuses of the type which fix a toner image on the support material by way of heat, a considerable portion of the drawn power is consumed by the fixing unit. The fixing unit ensures that a toner image adheres firmly to the support material by heat or by a combination of heat and pressure.
The energy consumption of a fixing unit can be reduced by generating heat in the fixing unit only when such heat really is required, i.e. at the time that toner really has to be fixed on a receiving sheet. This requires a fixing apparatus which can respond rapidly. Instant fixing units having a small heat capacity are suitable for this purpose. A description of such a fixing apparatus can be found in U.S. Pat. No. 4,355,225 to Marsh.
However, to obtain a good result, the fixing unit must be able to retain a specific temperature accurately during fixing. This necessitates accurate power control. Such accurate control is made possible by using an electronic switching element, such as a thyristor, triac or solid state relay.
A problem with such circuits is the formation of higher harmonics due to steep slopes in the waveform at the switching times, resulting in contamination of the mains. An example of a main is a power line that usually terminates in a wall socket and into which, usually, is plugged the reproduction apparatus. It is known to avoid these higher harmonics by switching at the times when the instantaneous voltages cross the zero-axis. The power supply can then be controlled by passing or blocking half periods in a suitable way. Such a power control circuit is described in U.S. Pat. No. 4,377,739 to Eckert, Jr. et al.
However, as a result of the pulsed consumption of large amounts of power taken from the mains, there are associated pulsating heavy currents drawn from the mains, which cause voltage variations to occur on the mains. These voltage variations on the mains cause flicker. Flicker is defined as "an impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time", in the International Standard CEI/IEC 1000-3-3 (the Flicker Standard).
Flicker is annoying to the user and is manifest by the fact that lamps which are connected to the mains, to which the reproduction apparatus is also connected, start flickering. The Flicker Standard describes two quantities by which flicker is characterised: the "short term flicker indicator" P.sub.st and the "long term flicker indicator" P.sub.lt. The first relates to the intensity (severity) of the flicker evaluated over a short period (a few minutes), and the second relates to the intensity (severity) of the flicker evaluated over a longer period (a few hours).
Flicker can be reduced by switching a solid state relay (SSR), not at the zero-cross times, but by applying phase angle control, i.e., phase cutting. However, this causes unwanted radiation. The above considerations also apply to other loads in a reproduction apparatus which draw high power from the mains, for example a paper preheating unit.